This invention relates to electrical equipment used for conditioning electric power and, more particularly, to methods of controlling such equipment.
Active filters and power line conditioners utilize pulse width modulated (PWM) inverters with high frequency switching. In general, these applications require the inverter to supply harmonic and transient currents to a node on the power line with the objective of maintaining either a sinusoidal fundamental current on the line feeding the node, or a sinusoidal voltage at the point of connection, or both of these. Modern inverters can operate at quite high power levels with switching frequencies as high as 20 kHz. They are thus intrinsically capable of producing output currents of up to about the 40th harmonic of 60 Hz.
In the early development of these systems, the approach used was to establish closed loop control of the inverter currents with the highest possible bandwidth. Each event on the power line that caused the controlled quantity to deviate from the desired sine wave would then produce a corrective response from the controller. This is a reasonably effective approach, but it cannot completely eliminate any harmonics because of the limited dynamic response of the current controller. In the case where the current controlled loop is referenced by an outer voltage control loop, the ability of the voltage controller to reduce harmonics may be seriously limited.
Vector control techniques have been used in prior art motor control systems. In a vector control system, a controlled quantity (such as a three phase current in which the individual currents sum to zero) is represented by a single vector. That vector is then transformed onto a synchronously rotating frame of reference to produce a dc signal. The dc signal can then be integrated and subjected to an inverse transformation to produce an output signal which is used as a control signal to adjust the controlled quantity.
While prior art rotating frame controllers are very effective at tracking a single balanced set of three phase sine waves with zero error, they typically behave poorly in response to additional components of different frequency or phase sequence, due to system non-linearities. This invention seeks to apply a rotating frame control technique to systems such as active filters and power controllers which are subject to harmonic interference on the controlled power line.